Page 535 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid and Diuretic Therapy in Heart Failure 523

TABLE 21-2 Diuretics
Primary
Diuretic Site of Mechanisms Adverse Effects in
Class Examples Action of Action Dogs and Cats

Carbonic Acetazolamide Proximal Inhibit membrane and Metabolic acidosis
anhydrase tubules cytoplasmic carbonic
inhibitors anhydrase
Loop Furosemide Thick Block Na-K-2 Cl Volume depletion, azotemia,
diuretics Bumetanide ascending cotransporter (symport) hypokalemia, hypomagnesemia,
Torsemide loop of hyponatremia, ototoxicity
Ethacrynic acid Henle
Thiazides Hydrochlorothiazide Distal Block the Na-Cl As described for loop diuretics
convoluted cotransporter (symport)
tubules
Thiazide-like Chlorthalidone Potential for hyponatremia
diuretics Metolazone Ventricular arrhythmias
(from hypokalemia)
Potassium- Triamterene Late distal Inhibit renal epithelial sodium Hyperkalemia
sparing Amiloride tubules channels (triamterene,
diuretics amiloride)
Aldosterone Spironolactone Collecting Inhibit mineralocorticoid
antagonists Eplerenone ducts receptors (spironolactone,
eplerenone)

Diuretic sites of action

Hypotonic

Thiazides H +
Isoto nic Amiloride
triamterene
-
Cl
Carbonic Na +
anhydrase Loop
inhibitors diuretics
Na + Spirono-
+ +
H O Na K lactone
2 -
2Cl Na +
+
Na pump
K +
Dopamine
agonists
(DA ) Hypotonic Impermeable H O with
1
2
H O to H O ADH
2
2
Osmotics Aquaretics
Figure 21-5 Renaleffectsofdiuretics.Loopdiureticssuchasfurosemidearemostcommonlyusedintreatmentofcongestiveheartfailure(CHF).
Spironolactone works in the distal nephron and is therefore a relatively weak diuretic; however, it also demonstrates cardiac-protecting properties.
In the concept of “sequential nephron blockade,” a loop diuretic would be combined with a thiazide and spironolactone or eplerenone to prevent
solute and water reabsorption at multiple levels. (From Opie LH, Gersch BJ. Drugs for the heart, 6th ed. Philadelphia: WB Saunders, 2005.)
First, the effectiveness of a diuretic depends on the ability effective. However, in severe chronic CHF, the more dis-
of cells distal to the site of diuretic action to reabsorb tal tubular cells can increase their reabsorption of sodium
sodium and water. Initially in CHF, loop diuretics, which and water and overcome the effects of the diuretic. 141
act on the thick portion of Henle’s loop, are highly This problem can be counteracted with additional